Revealing Decoherence Dependence of the Bright A Excitons on the Energy Positions of the Intervalley Dark Excitons in WS ₂ Semiconductors

Distinctive valley structures of the energy band in two-dimensional (2D) transition metal dichalcogenides (TMDs) offer various potential momentum-forbidden dark excitons, which significantly alter the decoherence of bright direct <i>KK</i> excitons, mainly via phonon-assisted intervalley scatterings. Nevertheless, the energy position influence of the intervalley dark excitons on the bright exciton decoherence has been rarely investigated. Herein, we report on a systematic combined study of optical spectroscopic experiments and quantum theory calculations for demonstrating the subject. At room temperature, external biaxial strain was applied to mono-, bi-, and trilayer WS₂ flakes; meanwhile, reflectance spectra were measured, followed by a quantum Rabi theory calculation to determine the homogeneous line widths of the bright <i>KK</i> excitons. It is unraveled that the homogeneous broadenings of all three flakes show non-monotonic dependence on strain: Decrease first and then turn to increase. Furthermore, the turning strain becomes larger for the thicker flakes. In order to quantitatively interpret the phenomenon, various individual contributions to the homogeneous broadening were calculated with the variational <i>Ansatz</i>, exhibiting an almost linear relationship with external strain. In particular, the energy positions of the intervalley dark excitons were revealed as an important factor tuning the bright excitonic decoherence. Enhancement coefficients of the Γ<i>K</i>, <i>K</i>Λ, and <i>K</i>Λ<i>'</i> intervalley scatterings, namely, ξ_Γ<i>K</i>, <i>ξ</i>_<i>K</i>_Λ, and <i>ξ</i>_<i>K</i>_Λ<i>'</i>, were thus defined and inferred to be approximately -0.1, -0.34, and -0.4, respectively. These findings demonstrate the important roles of the intervalley dark exciton energy positions in bright exciton decoherence, offering insights into the many-body physics in 2D semiconductor systems.